1. Field of the Invention
The present invention generally relates to a full-bridge soft switching inverter and a driving method thereof and, more particularly, to a full-bridge soft switching inverter and a driving method using zero-voltage switching and conducting a circular current in a circuit including two n-channel metal-oxide-semiconductor field-effect transistors (MOSFET's) so as to drive a load such as a cold cathode fluorescent lamp (CCFL) or a signal output device.
2. Description of the Prior Art
Soft switching has become a mainstream technique in the power control IC industry due to its capability in handling signals at high frequencies with low switching loss and high efficiency.
A conventional full-bridge soft switching inverter is as shown in FIG. 1. The full-bridge soft switching inverter generally comprises four n-channel MOSFET's QAN, QBN, QCN, QDN and a transformer TX. More particularly, the transistors QAN and QCN are coupled to an input voltage Vin, respectively, and the node VAB between the transistors QAN and QBN and the node VCD between the transistors QCN and QDN are coupled to the primary side of the transformer TX with its secondary side coupled to a load L. The load L is also coupled to the negative terminal of a first diode D1 and the positive terminal of a second diode D2. The positive terminal of the first diode D1 is grounded and the negative terminal of the second diode D2 is grounded through a feedback resistor RS.
FIG. 2 is another conventional full-bridge soft switching inverter and driving signals thereof. The full-bridge soft switching inverter in FIG. 2 is almost the same as the one in FIG. 1 except that the transistors QAP and QCP are p-channel MOSFETs.
FIG. 2 also shows the gate driving signals for the transistors QAP, QBN, QCP, and QDN. In FIG. 2, the first gate driving signal Drive_A and the second gate driving signal Drive_B are substantially in-phase and their duty cycles are approximately 50%. In order to insure zero-voltage switching, the duty cycle of the second gate driving signal Drive_B is slightly smaller than that of the first gate driving signal Drive_A, so that a period of turn-on dead time is available between the first and the second gate driving signals. Moreover, the third gate driving signal Drive_C and the fourth gate driving signal Drive_D are substantially in-phase and their duty cycles are approximately 50%. In order to insure zero-voltage switching, the duty cycle of the fourth gate driving signal Drive_D is slightly smaller than that of the third gate driving signal Drive_C, so that a period of turn-on dead time is available between the third and the fourth gate driving signals.
The full-bridge soft switching inverter in FIG. 2 conducts circular currents in a circuit including the transistors QAP and QCP and a circuit including the transistors QBN and QDN. A p-channel MOSFET is inferior to an n-channel MOSFET in turn-on characteristics such as carrier mobility, turn-on resistance and reaction time, which adversely affects switching efficiency and operation speed of a full-bridge soft switching inverter.
Therefore, there is need in providing a full-bridge soft switching inverter and a driving method thereof so as to drive at least a load such as cold cathode fluorescent lamp (CCFL) or a signal output device at high frequency with low switching loss and high efficiency.